The present invention concerns a device and method for fracturing a geological reservoir of hydrocarbons, a method for producing hydrocarbons and a method for calibrating the device.
For the production of hydrocarbons the permeability and/or porosity of the constituent material of the reservoir has an influence on the production of hydrocarbons, and in particular on production rate and profitability. This is notably recalled in the article “Porosity and permeability of Eastern Devonian Shale gas” by Soeder, D. J., published in SPE Formation Evaluation, in 1988, vol. 3, no 1, pp. 116-124, which reports on the study of eight samples of Devonian shale gas in the Appalachian Basin. This article explains in particular that the production of this shale gas encounters difficulty due to the low permeability of the reservoir (i.e. the constituent material of the reservoir).
Various techniques are available to facilitate hydrocarbon production rates, in particular in a scarcely permeable and scarcely porous reservoir. These techniques entail the static or dynamic fracturing of the reservoir.
Static fracturing relates to the targeted dislocation of the reservoir using the injection of a fluid under very high pressure intended to split the rock. Fracturing is obtained by mechanical “stress” provided by hydraulic pressure obtained by means of a fluid injected under high pressure from a wellbore drilled on the surface. The terms “frac jobs”, “frac'ing” or more generally “fracking” are used, or “massive hydraulic fracturing”. U.S. Patent Publication No. 2009/044945 in particular describes a static fracturing method such as described above.
Static fracturing has the disadvantage that the fracturing of the reservoir is generally unidirectional. Therefore only the hydrocarbon contained in the portion of reservoir around a deep, highly localised fissure can be produced more rapidly.
To obtain more diffuse splitting, dynamic fracturing, or electric fracturing, has been introduced. For electric fracturing, an electric arc is generated in a well drilled in the reservoir (typically the production well). The electric arc induces a pressure wave which fractures the reservoir in all directions around the wave and therefore increases the permeability of the reservoir.
Several documents report on electric fracturing. For example U.S. Pat. No. 4,074,758 presents a method whereby an electro-hydraulic shock wave is generated in a liquid in the wellbore for better oil recovery. U.S. Pat. No. 4,164,978 suggests having the shock wave followed by an ultrasound wave. U.S. Pat. No. 5,106,164 also describes a method for generating a plasma explosion to fracture the rock, but for a hole of shallow depth and for a mining application and not for the production of hydrocarbons. U.S. Pat. Nos. 4,651,311 and 4,706,228 present a device for generating an electric discharge with electrodes in a chamber containing an electrolyte wherein the electrodes are not subject to erosion by the discharge plasma. Document WO 2009/073475 describes a method for generating an acoustic wave in a fluid medium contained in a well using a device comprising two electrodes between an upper packer and a lower packer defining a confined space. This document describes that the acoustic wave is held in “non-shockwave” state to improve fracturing, without explaining however the differences between “ordinary” acoustic wave and “shock” wave.
None of these documents produces fully satisfactory fracturing of the reservoir. There is therefore a need for improved fracturing of a hydrocarbon reservoir.
For this purpose, a fracturing device is proposed to fracture a geological hydrocarbon reservoir wherein the device comprises two packers defining therebetween a confined space in a well drilled in the reservoir, apparatus for regulating the temperature of a fluid in the confined space, a pair of two electrodes arranged in the confined space and an electric circuit to generate an electric arc between the two electrodes. The circuit comprises at least one voltage source connected to the electrodes and an inductor coil between the voltage source and one of the two electrodes.
According to examples, the device may comprise one or more of the following characteristics:                the temperature regulating apparatus regulates the temperature of the fluid to optimise the energy yield of the pre-discharge phase when generating an electric arc;        the temperature regulating apparatus holds the temperature of the fluid at a value between 45 and 67° C., preferably higher than 50° C. and/or lower than 62° C.;        the device further comprises apparatus for regulating the pressure of the fluid substantially at atmospheric pressure;        the temperature regulating apparatus comprises a fluid cooling system;        the inductor coil has adjustable inductance, preferably between 1 μH and 100 mH, more preferably between 10 μH and 1 mH;        the distance between the two electrodes is adjustable, preferably between 0.2 and 5 cm, more preferably between 1 and 3 cm;        the voltage source comprises a capacitor having a capacitance higher than 1 μF, preferably higher than 10 μF;        the capacitance of the capacitor is adjustable, preferably lower than 1000 μF, more preferably lower than 200 μF;        the circuit further comprises a Marx generator and ferrites forming a saturable inductor on a pathway leading the capacitor directly to the inductor, the ferrites being saturated once the Marx generator is discharged;        the capacitor is separated from the inductor by a spark gap primed by a pulse generator;        the electrodes have a radius between 0.1 mm and 50 mm, preferably between 1 mm and 30 mm;        the device comprises a release system; and/or        the device comprises several pairs of electrodes.        
A fracturing method is also proposed to fracture a geological reservoir of hydrocarbons. The method comprises electric fracturing of the reservoir by generating an electric arc with the above circuit and simultaneously regulating the temperature of a fluid in the confined space of the device. A hydrocarbon production method is also proposed comprising the fracturing of a geological reservoir of hydrocarbons using the above method.
Also proposed is a method to calibrate the temperature regulating apparatus of the above device. The calibrating method comprises the steps of providing the device, determining a pre-breakdown voltage over and above which the electric arc is generated, measuring a breakdown voltage at the terminals of the electrodes and a breakdown time as a function of the temperature of the fluid, by applying the pre-breakdown voltage and causing the temperature of the fluid to vary, then inferring from the preceding step the energy yield of the pre-discharge phase as a function of the temperature, and defining a target temperature or temperature range for the temperature regulating apparatus as a function of the maximum energy yield inferred at the preceding step.